Blepharitis is an abnormal condition wherein the tears produced contain an excess of lipids (the oily ingredient in natural tears) and, in some cases, contain an irritating oil as well. As explained hereinafter, this oil ingredient serves to prevent evaporation of the aqueous layer that wets the corneal epithelium of the eye and helps spread the aqueous layer over the normally aqueous-resistant cornea during a blink. If excess oil is present, the lipid layer will tend to adhere to the cornea itself. If the eye is unable to clear this oil from the surface of the cornea, a "dry" area occurs on the cornea since the aqueous layer is unable to hydrate this area.
"Dry eye" can also occur because of a diminution of the quantity of tears produced and distributed through the lachrymal ducts, as well as the previously described decrease in the stability of the tear film produced. "Dry eye" acts to decrease visual acuity; produces discomfort; and eventually, if allowed to remain untreated and uncorrected, may result in permanent damage with degradation of the exposed ocular tissues, a complete breakdown of corneal tissue necessitating, in the extreme, corneal transplants.
Various compositions for treating "dry eye" have been proposed and put into use over the years. For example, the treatments employed by ancient Greek physicians for this condition dominated medical practice throughout the Middle Ages and into the nineteenth century. The selection of components for ancient collyria, or for any of the eye treatment preparations of the time, suggests either an instinctive or empirical knowledge of the composition of tears and tear films. Egg whites, very rich in albumen (a major tear protein), and goose fat, a lipid admixture, which, like meibomian lipids, becomes fluid at temperatures approximating normal body temperatures, have been used.
Use has also been made of substances which serve to induce a measure of irritation, presumably to induce reflex tearing. Such substances as alcohols, acetic acid values of vinegar, onion fermentates and the like have been utilized in this approach. Obviously, such methods are less than totally acceptable.
Other solutions offered for the alleviation of "dry eye" in more recent years, i.e., during the 19.sup.th and early 20.sup.th century, have included aqueous solutions of common table salt, glycerol, various oils and isotonic solutions of various salts, known as Ringer's and Locke's solutions.
Approximately forty years ago, the employment of aqueous solutions of inert, substituted cellulose ethers, such as methyl cellulose, was proposed, and such formulations are currently in use. Other substituted cellulose ethers, such as hydroxyethyl cellulose, hydroxypropyl methyl cellulose, and hydroxypropyl cellulose, have been subsequently utilized as polymeric components in artificial tear formulations. Each of these polymeric materials imparts high viscosity to the tear formulations, even when employed in relatively low concentrations. It has been this impartation of high viscosity to the formulations which is believed to prolong retention time of the tear substitutes in the fornices and over the preocular surface.
Recently, in U.S. Pat. No. 4,421,748 to Trager et al., a sterile hypotonic solution of 1-20 percent lecithin, preferably lecithin sulfate, and 0.1-20 percent of a cellulose-containing viscosity-adjusting agent, preferably hydroxyethyl cellulose, has been disclosed as an artificial tear formulation. Such compositions possess surface tensions approximating those of natural tears; and if not, Trager et al. recommended the addition of 2-10 percent weight or volume of a nonionic surfactant such as polyoxyalkylene oleic ester of asorbitol anhydride to control surface tension of the artificial tear formulation.
It is an object of this invention to prevent "dry eye," or, if "dry eye" has occurred, to provide an improved formulation that quickly thereafter overcomes "dry eye" and permits the natural tears to operate in the manner that nature intended. It is a further object to avoid the use of an oil to overcome "dry eye" since adding an oil, other than the amount of oil in a natural tear, tends to overcome "dry eye" only temporarily. Other objects will be apparent hereinafter.